1. Field of the Invention
The present invention relates to the field of image recording and reproducing using apparatus such as a video camera and video tape recorder. More particularly, the present invention is directed to a method of and control circuit for controlling the compression of image signals for recording and for controlling the expansion of compressed image signals for reproduction. The present invention may be used in systems where a plurality of video cameras are used to produce a plurality of image signals for recording on one or more video tape recorders.
2. Description of the Related Art
In a number of systems, such as surveillance systems and the like, a plurality of video cameras are employed to observe an area for a long period of time. Typically, each of the cameras is used to observe a different portion of the area. When a permanent record of the surveillance is desired, the plurality of video cameras produce image signals that are recorded on a plurality of video tape recorders. As the surveillance is made over a long period of time, a great number of video tapes are required to continuously record the image signals produced by the video cameras. In order to reduce the number of video tapes required for such recording, there has been proposed a time lapse video tape recorder that is capable of recording image signals produced over a long period of time on a single video tape. The time lapse video tape recorder operates by intermittently recording a portion of each image signal produced by each of the plurality of video cameras. This intermittent operation allows the time lapse video tape recorder to record, for example, anywhere between 24 hours and 960 hours of surveillance image signals using a video tape of 120 minutes in length. The time lapse video tape recorder is much like the conventional video tape recorder used for general purposes, such as the well known VHS-type video tape recorder. As a result, the time lapse video tape recorder is limited to recording 30 frames of image data per second, each frame having two fields (i.e., 60 fields per second). Thus, if the video tape length is limited to 120 minutes, the video tape recorder is limited to recording 216,000 frames of image data (i.e., 120 minutes.times.60 seconds/minute.times.30 frames/second). Therefore, the recording interval (i.e., the time which lapses between intermittent recording of image signals) for recording 216,000 frames (or 432,000 fields) over a period of, for example, 480 hours is as follows. ##EQU1##
As it can be seen, the time lapse video tape recorder pauses for four seconds after it records each field of data from a given one of the plurality of video tape recorders used in the surveillance system. Having a pause of four seconds between recording causes serious problems in the surveillance system because all of the image information taken by the video camera during the nonrecording period will be lost. As a result, no surveillance during the nonrecording period is possible using the video tape.
In addition, it is very difficult to record the plurality of image signals onto separate video tapes for each of the plurality of video cameras because the apparatus required to perform the operation would be so large that it would be uneconomical to do so. To overcome this problem, there has been proposed a switching device that sequentially selects one of a plurality of channels in multiplexing fashion so that the image signals produced by the plurality of video cameras can be recorded on one or a reduced number of video tape recorders. This switching device is effective to reduce the number of video tapes required to record the surveillance image signals over a long period of time (e.g., 480 hours); however, the switching device requires a long recording interval for each camera, which may result in serious problems for the surveillance system. For example, assuming that the image signals from 8 video cameras are recorded on one video tape recorder during 480 hours of continuous surveillance using the automatic sequential switching device, the recording interval for each camera becomes 32 seconds (i.e., 4 seconds (as calculated above).times.8 cameras). Thus, a video camera fixed on a portion of secured area would only record an image every 32 seconds. This recording interval magnifies the problem identified above in that the recording interval is too long to adequately provide surveillance of an area. For example, an intruder appearing within the viewing area of a camera will go undetected as long as he does not appear within the viewing range of any camera for more than a few seconds (as much as 32 seconds) at a time.